A long-term Warming Experiment to elucidate Soil Organic Matter dynamics in a subarctic forest Ecosystem (AWESOME)

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Climate change is leading to a pronounced warming of soils, which is accelerating and increasing the decomposition of soil organic matter (SOM). This could become one of the strongest Climate-Carbon Cycle feedback mechanisms, since this process could emit large amounts of CO2. However, it is still highly uncertain, if soils in all ecosystems will become a CO2 source due to global warming. The warming response of the various biogeochemical processes involved in SOM turnover are not understood in sufficient detail. This is also due to experimental difficulties and barriers related to soil warming. Especially long-term and deep-soil warming observations are scarce, which hampers reliable estimates of new steady-state situations after the onset of warming. Also tipping points related to warming intensity cannot be investigated when only one level of experimental soil warming is applied. In the AWESOME project, we will make use of a natural, geothermal soil warming gradient in the Canadian Yukon Territory to gain essential insights on the complex interplay of autotrophic and heterotrophic organisms and the mineral soil matrix as affected by increasing temperature. First results showed that organic carbon was reduced by up to 22% in the geothermally warmed soils under subarctic deciduous forest, while the soil nitrogen stock remained constant. However, nitrogen was found to be more stabilized in the mineral associated organic matter fraction with increased warming. Four different warming intensities with a total temperature range of approximately 8°C are already established and an in-situ microcosm experiment with labeled litter was initiated in summer 2019. The ultimate aim of the project is to establish a new conceptual framework of organic matter cycling in century-scale warmed subarctic forest soils that could potentially tested in other ecosystems in the future. Changes in vegetation and carbon inputs, microbial physiology, community composition and adaptation as well as organic matter quality will be the focus of the project. Warming effects on soil hydrology (moisture) and mineralogy (weathering) will be evaluated as explanatory variables across the whole warming gradient. In cooperation with several partners, by using state of the art methods in soil science and microbiology, as well as an ideal research site, the AWESOME project is expected to bring about significant progress in a topic that will be highly relevant in the 21st century. The results will be compared to findings from another geothermal warming experiment (on Iceland) to elucidate differences and commonalities and come up with a more generic framework of long-term warming effects on biogeochemical cycling in soils.